**Background:** Antimicrobial resistance (AMR) is a global health threat, and Listeria monocytogenes is a leading cause of death from foodborne pathogens, with listeriosis mortality rates reported between 20% and 30%. In the EU in 2021, L. monocytogenes caused 2183 illnesses, 923 hospitalizations, and 196 deaths. Ampicillin (with or without gentamicin) is the first-line treatment for invasive listeriosis. Although CLSI does not provide disc diffusion breakpoints specifically for L. monocytogenes, EUCAST provides criteria for five antibiotics using MH-F medium. The authors note that repeated surveillance studies are justified because antimicrobial resistance patterns can change over time and every microbial collection adds unique data.
**Methods:** A total of 153 L. monocytogenes isolates were collected from meat food samples (raw and processed) and meat-processing environments (both food-contact and non-contact surfaces) in Poland between October 2020 and November 2021. Species identity was confirmed by RFLP-PCR and multiplex PCR. Antimicrobial susceptibility was tested using the disc diffusion method on Mueller–Hinton agar (Oxoid) with 10 antibiotic discs: ampicillin (10 µg), chloramphenicol (30 µg), ciprofloxacin (5 µg), erythromycin (15 µg), gentamicin (10 µg), penicillin (10 IU), streptomycin (10 µg), sulfamethoxazole/trimethoprim (1.25/23.75 µg), tetracycline (30 µg), and vancomycin (30 µg). Incubation was at 37°C for 18±2 h. Zones of inhibition were measured three times per replicate, with two technical repeats. Results were interpreted using CLSI breakpoints for Enterococcus spp. where available, and Enterobacterales breakpoints for gentamicin, streptomycin, and sulfamethoxazole/trimethoprim. For the ten isolates with reduced ciprofloxacin susceptibility, RAPD-PCR (OMP-01 primer) and REP-PCR (REP 1R-I and REP 2-I primers) fingerprinting were performed, and band patterns were analyzed with GelClust software (UPGMA, Jaccard coefficient).
**Key Results:** Of the 153 isolates, 143 (93.5%) were susceptible to all ten tested antibiotics. No isolate was resistant to more than one antibiotic. For nine antibiotics (ampicillin, chloramphenicol, erythromycin, gentamicin, penicillin, streptomycin, sulfamethoxazole/trimethoprim, tetracycline, vancomycin), 100% of isolates were classified as sensitive. The only antibiotic to which any isolate showed reduced susceptibility was ciprofloxacin: 10 isolates (6.5%) were classified as intermediate (zone diameters 16–20 mm), and none were resistant. The histogram for ciprofloxacin showed a clear bimodal distribution, with the intermediate subpopulation visibly separated from the sensitive majority. All ten ciprofloxacin-intermediate isolates originated from food-processing environmental swabs (e.g., meat injector, transport box, ham wrapper, exit door, worker hands/gloves, boards, boxes, metal mats). Three were sampled on the same day in one plant, and seven on the same day in another plant. All ten belonged to genoserotype VIb (serotypes 4b, 4d, 4e). RAPD- and REP-PCR fingerprinting showed that the ten isolates had very similar band patterns, clustering together with a within-group Jaccard distance coefficient range of 0.097–0.245. Three ciprofloxacin-sensitive comparator isolates were clearly separated from this cluster (between-cluster distance coefficient 0.424). The authors conclude that the ten isolates likely represent a single contamination event in the processing environment.
**Clinical Implications:** The study provides reassuring evidence that L. monocytogenes from Polish meat products and processing environments remains largely susceptible to antibiotics used in human listeriosis treatment, particularly ampicillin (the drug of choice). The 6.5% rate of intermediate ciprofloxacin sensitivity is low and consistent with a recent Polish study (5% intermediate). However, the finding that all ten intermediate isolates were of the same genoserotype and clustered by fingerprinting suggests that reduced ciprofloxacin sensitivity may be linked to a specific clonal lineage, which warrants further genomic investigation. The authors note that applying different interpretive criteria (e.g., EUCAST breakpoints) would have classified three isolates as resistant to trimethoprim/sulfamethoxazole, two of which were also among the ciprofloxacin-intermediate group. The study's limitations include the inherent variability of the disc diffusion method (where a 1 mm difference can change classification) and the absence of MIC determination. The authors recommend continued surveillance and suggest that more sensitive methods (PFGE or WGS) could clarify whether the ciprofloxacin-intermediate isolates are clones or represent a genotype-specific phenotype.